DESCRIPTION: Hair cells use an adaptation mechanism to ensure that their transduction channels operate at their point of optimal sensitivity. Determination of the molecular identity of proteins responsible for adaptation is important for several reasons: First, such identification should assist us in attempts to discover other members of the transduction apparatus. Second, because hair cells are such a scarce source of material to study, identification of the responsible gene will permit us to express its protein in abundance for biochemical and biophysical studies. Finally, should human deafnesses derive from mutated genes encoding the adaptation motor or associated proteins, future gene-replacement therapies employing a functional gene could conceivably correct such deficits. The best candidate for the hair cell's adaptation motor is myosin IB, an unconventional myosin isozyme that is found at stereociliary tips. Because hair cells are so scarce and because hair-bundle displacements directly open transduction channels, proof that myosin IB is the adaptation motor using biochemical methods is extremely difficult, if not impossible, to obtain. To test whether myosin IB is indeed the adaptation motor, we will instead eliminate its gene from the mouse's genome, and study the consequences on auditory and vestibular function that ensue. Because a complete knockout of myosin IB's gene may have broad effects that may confound our ability to assess its specific role in adaptation, we will also express in transgenic mice mutant myosin genes that should have predictable effects on adaptation. This proposal draws on the strengths of three laboratories: the Mercer laboratory, with extensive experience in mouse genetics and molecular cloning, and the Gillespie and Corey laboratories, with strengths in biochemical analysis and physiological assessment of adaptation in both amphibian and mammalian hair cells. The experiments proposed here will directly test whether myosin IB is indeed the hair cell's adaptation motor.